High-precision positioning instruments are used, for example, in machining tools, lithography equipment for semiconductor wafer processing or liquid crystal display devices, or the like. A control system drives a stage or stages of the positioning instrument in accordance with a defined path. The path may have one direction, e.g., the X coordinate direction, or two directions, e.g., the X and Y coordinate direction s.
Typically in lithography equipment for semiconductor wafer processing, a first stage is used to position the subject plate (wafer) in two dimensions, while a separate stage is used to synchronously position the mask (reticle). The stages are moved relative to a source of radiant energy and a projection lens to focus the energy as well as the base structure that supports the stages. During exposure the stages may be moved either in a constant velocity "scanning" pattern or in a "step-and-repeat" pattern.
In a high-precision positioning instrument the stages must be moved in a synchronous fashion. Where extreme precision is required, such as in a microlithographic system that produces images on the sub-micron scale, any misalignment of the stages will result in defects in the exposed image. Misalignment of the stages is known as synchronous error.
Conventional positioning instruments typically control the stages using a velocity feedback system. Thus, the controller of conventional positioning instruments emphasize velocity control over control of the position of the stage. A positioning instrument typically uses velocity information or position information obtained from a non-center of gravity location on the stage. For example an interferometer measurement system uses mirrors located on the sides of the stage. However, a measurement taken with a mirror on the side of the stage will increase or decrease, while the position of the center of gravity of the stage may not change, when the stage is rotated slightly, as illustrated in FIG. 4. Thus, a velocity error, as well as a positioning error, is possible where a stage is controlled with information derived from the non-center of gravity location. Consequently, a stage being controlled with information obtained from a non-center of gravity location may develop a synchronous error.
An additional source of synchronous error is caused by a control system that uses the absolute position of the stage as feedback, for instance by way of a linear encoder. Because the positioning system generally uses a single precision 32 bit floating point CPU, digital signal processor, or a micro-processor, the accuracy of the measurement is limited to only the 23 bits for the significant digits "significand." Thus, where the absolute position of the stage is used as feedback in a control system, the floating point unit will represent the absolute position of the stage in 23 bits. Consequently, if the absolute position of the stage is larger than 23 bits, accuracy of the position is lost.
In addition, conventional control systems typically convert the trajectory command for a stage into a force through the use of a proportional-integration differentiation (PID) device as is well known in the art. However, a conventional PID device typically permits the synchronous error to accumulate during acceleration periods. After the stage stops accelerating the synchronous error is reduced. Consequently, a period of time must elapse after the acceleration period before exposure may begin because the synchronous error must be eliminated. This time is known as the settling time of the system and is a limitation on the throughput of the system.
Thus, a position control system that drives the center of gravity of the stage with a level of accuracy that is not affected by the absolute position of the stage is needed to reduce synchronous error. Moreover, a position control system that limits the accumulation of synchronous error thereby reducing settling time is needed.